OneFastShop B58 Catless Downpipe | Performance Gains, Sound, and Install Overview

The BMW B58 is one of the best turbocharged inline-sixes BMW has ever built - smooth, torque-rich, and famously overbuilt, with a closed-deck block and robust internals that shrug off far more power than they make from the factory. It powers everything from the F30 340i to the G-Series M340i and M440i, and it shares its core with the Toyota Supra A90. From the factory it is already strong, and it responds extremely well to bolt-on modifications. The catless downpipe is usually one of the first upgrades B58 owners consider - and for good reason.

This guide covers what a downpipe actually does, the real-world power gains you should expect, the trade-offs nobody likes to mention (sound, smell, check engine lights), fitment notes, and whether the OneFastShop B58 4.5” Catless Downpipe is the right call for your build.

What a Downpipe Does on the B58

The downpipe is the section of exhaust that bolts directly to the turbocharger’s turbine housing and carries hot gases into the rest of the exhaust. The factory B58 downpipe contains a dense catalytic converter that is there to meet emissions and noise regulations. That cat is the single biggest flow restriction between the turbo and the tailpipe - a honeycomb of coated ceramic that exhaust gas has to squeeze through on its way out.

Why does that matter so much on a turbo car specifically? Because anything that restricts flow right behind the turbo creates backpressure, and backpressure works directly against the turbine wheel. The turbine is trying to spin freely on the exhaust energy; a restriction downstream makes it fight to push gas out, which slows spool and limits how much the turbo can flow once it is up. Removing that restriction lets exhaust gases leave the turbine far more freely. The practical effects are lower backpressure, lower exhaust gas temperatures (EGTs), and quicker turbo spool. Lower EGTs in particular matter on a tuned car, because heat is what limits how much timing and boost you can safely run - cooler exhaust gas gives the tune more thermal headroom to make power without creeping toward knock. So a downpipe is not just a power part, it is a reliability-enabler for a tuned car.

Real Power Gains: Honest Numbers

This is where a lot of marketing gets carried away, so here is the honest version. On a B58 running a supporting tune, a catless downpipe typically adds in the range of 20 to 30 wheel horsepower and 25 to 30 lb-ft of torque. Some setups see a bit more, some a bit less, depending on the tune, fuel, and the rest of the exhaust.

Two things are critical to understand:

• The gains require a tune. Bolting on a downpipe with no tune leaves most of the potential on the table. The real power comes from the ECU being able to run more boost and timing once the restriction is gone - the downpipe opens the door, and the tune is what walks through it.
• Peak horsepower is not the whole story. A lot of the benefit shows up as faster spool, stronger mid-range, lower EGTs, and a more responsive feel rather than a huge peak number. If you are chasing only the dyno headline figure, manage your expectations. If you want the engine to breathe better, run cooler under load, and feel more eager everywhere, this is a meaningful upgrade you notice every drive.

The Trade-Offs You Should Know About

A catless downpipe is the right choice for many builds, but it is not free of downsides. Being upfront about these is the difference between a happy install and buyer’s remorse.

Sound

Removing the cat makes the car noticeably louder and raspier, especially under load and at wide-open throttle - the cat acts as a secondary muffler, and taking it out uncovers the raw turbo-six character underneath. At idle the difference can be subtle. Paired with an aftermarket cat-back it becomes genuinely aggressive. Many enthusiasts love this; if you want a refined, luxury-quiet car, a high-flow catted downpipe is the better fit.

Highway Drone

Depending on the rest of your exhaust, cruising drone - that resonant hum at steady highway RPM - can increase. It is the most common regret for people who daily long highway miles. A quality valved cat-back designed for catless applications helps tame it considerably.

Smell and Emissions

With the catalytic converter gone, you will notice more raw fuel smell, particularly at stoplights and during cold starts, since the cat is what normally scrubs those hydrocarbons. A catless downpipe is intended for off-road and track use and will not pass a visual or sniffer emissions inspection in regions that require it. This is not fine print to skim - check your local laws before buying.

Check Engine Light

On a catless setup the ECU usually triggers a check engine light, because the downstream oxygen sensor expects to see a working catalyst and no longer does. This is normally resolved by the same tune that unlocks the power, which can code out the relevant readiness monitor. Plan on tuning and the CEL becomes a non-issue.

Construction and Build Quality

A downpipe lives in one of the hottest, highest-stress areas of the car - bolted to a turbine housing that glows under load and heat-cycles from cold to extreme every drive. Cheap pipes crack at the welds and warp at the flanges; material and construction are what separate a part you fit once from one you fight with. The OneFastShop B58 downpipe is built to be the former:

• Material: 304-grade stainless steel for corrosion and heat resistance.
• Tubing: 4.5” mandrel-bent piping, which keeps a consistent internal diameter through the bends instead of crushing and choking flow.
• Flanges: CNC-machined for accurate, leak-free sealing and proper alignment to the turbo and mid-pipe.
• Welds: Hand TIG-welded for strength and long-term durability under constant heat cycling.

Installation

The OneFastShop B58 downpipe is a direct bolt-on that uses the factory mounting points and hardware, with no cutting or welding required. It works on both left-hand and right-hand drive cars and integrates with the OEM exhaust or most aftermarket systems.

That said, downpipe access on the B58 is tight, and the job involves working around the turbo and getting to the factory clamps, usually from underneath with the car safely raised. Experienced DIYers can do it with hand tools and patience over an afternoon, but plenty of owners hand it to a shop - there is no shame in that given how cramped the area is. Either way, plan to follow up with a tune, and use a fresh gasket where the downpipe meets the turbo rather than reusing the old one.

Fitment: Check Before You Buy

The OneFastShop B58 4.5” Catless Downpipe fits B58-equipped BMW and Toyota models, including:

• Toyota Supra A90 (2020–2024)
• BMW 2 Series – M240i (2017–2024)
• BMW 3 Series – 340i (2016–2018), M340i (2020–2024)
• BMW 4 Series – 440i (2017–2024), M440i (2021–2024)
• BMW 5 Series – 540i (2017–2023)
• BMW 6 Series – 640i (2018–2019)
• BMW 7 Series – 740i, 745e (2016–2022)
• BMW 8 Series – 840i (2020–2024)
• BMW X3 M40i (2018–2024), X4 M40i (2019–2024)
• BMW X5 40i / 45e (2019–2024), X6 40i (2020–2022), X7 40i (2019–2022)
• BMW Z4 M40i (2020–2024)

European owners: later cars may be fitted with an OPF (gasoline particulate filter). OPF and non-OPF cars use different exhaust configurations, so confirm which one your car has before ordering - it is the single most common fitment mistake on this platform. If you are unsure, reach out and we will help you confirm before you buy.

Catless vs High-Flow Catted: Which Is Right for You?

This is the real decision for most B58 owners, and it comes down to how you use the car more than which makes a bigger dyno number.

Go catless if you want the most aggressive sound, the lowest EGTs, the best price, and you have a track car or live somewhere without emissions testing. You accept the smell and the louder cabin as part of the deal.

Go high-flow catted if you daily drive the car, want to keep it closer to street legal, dislike raw fuel smell, and prefer a deeper note without the loudest rasp. The important point: a high-flow catted downpipe gives up only a small amount of power versus catless on the B58, so you are not sacrificing much performance to keep a cat - for a daily-driven car, it is often the smarter long-term choice.

Is It Worth It?

For a tuned B58, a catless downpipe is one of the better value-per-dollar upgrades available. It lowers EGTs, sharpens spool, frees up real power with a tune, and completely transforms the car’s sound. As long as you go in understanding the trade-offs - louder, some smell, off-road use, tune required - most enthusiasts are very happy with the result. If you daily the car in an emissions area, weigh the catted option honestly; if it is a weekend or track car, catless is hard to beat for the money.

The OneFastShop B58 4.5” Catless Downpipe is in stock and ready to ship. Check current pricing and confirm fitment for your B58 here.

Frequently Asked Questions

Do I need a tune for a B58 catless downpipe?

Yes. A tune is effectively required. It unlocks the actual power gains and codes out the check engine light caused by removing the catalytic converter. Running a downpipe without a tune leaves most of the benefit unrealized.

How much horsepower does a B58 downpipe add?

With a supporting tune, expect roughly 20 to 30 wheel horsepower and 25 to 30 lb-ft of torque, along with faster spool and lower exhaust gas temperatures. Gains vary with tune, fuel, and the rest of the exhaust.

Will a catless downpipe throw a check engine light?

On a catless setup, usually yes, until you tune the car. The tune that adds power also disables the downstream catalyst monitor, which clears the light.

Is a catless B58 downpipe loud? Will it drone?

It is significantly louder than stock, especially under acceleration, with a deeper and raspier tone. Highway drone can increase depending on your cat-back. A valved exhaust designed for catless use helps reduce drone.

Catless or catted downpipe for a daily-driven B58?

For a daily driver, especially in an emissions-testing area, a high-flow catted downpipe is often the smarter choice - it keeps the car closer to street legal, reduces fuel smell, and gives up only a small amount of power versus catless on the B58. Catless is best suited to track and off-road cars where sound and lowest EGTs are the priority.

Is a catless downpipe street legal?

No. Removing the catalytic converter makes it off-road and track use only, and it will not pass emissions testing in regions that require it. Always check your local regulations first.